The present invention uses a substrate which, in the preferred embodiment includes a silicon layer. However, a deposited material, such as polysilicon or amorphous silicon, may also be used. Typically, these are semiconductor wafers, although it is possible to use other materials, such as silicon on saphire (SOS). Therefore, "wafers" is intended to refer to the substrate on which the inventive emitter tips are formed.
Flat panel displays have become increasingly important in appliances requiring lightweight portable screens. Currently, such screens use electroluminescent or liquid crystal technology. A promising technology is the use of a matrix-addressable array of cold cathode emission devices to excite phosphor on a screen.
The clarity, or resolution, of a field emission display is a function of a number of factors, including emitter tip sharpness, alignment and spacing of the gates which surround the tips, pixel size, as well as cathode-to-gate and cathode-to-screen potentials. The process of the present invention is directed toward the fabrication of very sharp cathode emitter tips.
A great deal of work has been done in the area of cold cathode tip formation. See, for example, the "Spindt" patents, U.S. Pat. Nos. 3,665,241, and 3,755,704, and 3,812,559 and 5,064,396. See also, U.S. Pat. No. 4,766,340 entitled, "Semiconductor Device having a Cold Cathode," and U.S. Pat. No. 4,940,916 entitled, "Electron Source with Micropoint Emissive Cathodes and Display Means by Cathodeluminescence Excited by Field Emission Using Said Source."
U.S. patent application Ser. No. 837,833, entitled "Method of Creating Sharp Asperities and other Features on the Surface of a Semiconductor Substrate," has the same assignee as the present application. It describes a worthwhile method to fabricate emitter tips, as well, but employs a significantly different approach than the process of the present invention.
In contrast to the above-cited methods, the process of the present invention employs dry etching (also referred to as plasma etching) to fabricate sharp emitter tips. Plasma etching is the selective removal of material through the use of etching gases. It is a chemical process which uses plasma energy to drive the reaction. Those factors which control the precision of the etch are the temperature of the etchant, the time of immersion, and the composition of the gaseous etchant.
Various papers refer to reactive ion etching (RIE) and orientation dependent etching (ODE) of silicon to form cathode emitter tips. These technologies rely on either expensive multiple deposition and evaporation steps, or dry etch processes bound by the isotropic etching characteristics of the process gases. For example, prior art dry etch processes limit the manufacturer to a height to width etch ratio of 1:1. To alter this 1:1 ratio to obtain an increased depth, a deeper mask would be required.